The semiconductor industry has been using single substrate (silicon wafer) processing chambers for some time because the chamber volume can be minimized, contamination of the substrate has been reduced, process control is increased and, therefore, yields are improved. Further, vacuum systems have been developed, such as described in Maydan et al, U.S. Pat. No. 4,951,601, that allow several sequential processing steps to be carried out in a plurality of vacuum processing chambers connected to a central transfer chamber, so that several processing steps can be performed on a substrate without its leaving a vacuum environment. This further reduces the possibility of contamination of the substrates.
Recently the interest in providing large glass substrates with up to one million active thin film transistors thereon for applications such as active matrix TV and computer displays has been heightened. These large glass substrates, generally of a size up to about 350.times.450.times.1 mm, require vacuum processing chambers for deposition of thin films thereon. The basic methods and processing chambers, e.g., plasma-enhanced chemical vapor deposition (PECVD), PVD, etch chambers and the like, are similar to those used for depositing layers and patterning thin films on silicon wafers. A practicable system that can perform multiple process steps on glass substrates is disclosed by Turner et al Serial No. 08/010,684 filed Jan. 28, 1993 in a copending application filed concurrently herewith entitled "VACUUM PROCESSING APPARATUS HAVING IMPROVED THROUGHPUT." However, because of the large size of the glass substrates, several problems have been noted in their handling and processing in vacuum processing chambers.
During processing, the edge and backside of the glass substrate must be protected from deposition. Borrowing from the semiconductor processing art, a deposition-masking ring (or in this case, a rectangle) or shadow frame is placed about the periphery of the substrate to prevent processing gases or plasma from reaching the edge and backside of the substrate in a CVD chamber for example. The susceptor, with a substrate mounted thereon, can have a shadow frame which will surround and cover several millimeters of the periphery of the front surface of the substrate and this will prevent edge and backside deposition on the substrate. If however, the shadow frame is not properly centered with respect to the substrate during processing, the amount of shadowing that occurs on each edge of the substrates will be unequal and unacceptable.
A factor complicating the alignment of the substrate to the susceptor is the following. For proper set-up, calibration, and debugging of the automated movement of a substrate into and out of a processing chamber, it is important to be able to execute these activities at room temperature. Therefore, the chamber components which provide support and alignment for the substrates must be sized and shaped to perform similarly at room temperature as at normal operating temperature. The susceptor or support for the large glass substrate, generally made of aluminum and which is heated resistively or otherwise, has a very large coefficient of thermal expansion or CTE (about 22.times.10.sup.-6 /.degree.C.) and thus increases in size by 0.72% when heated from room temperature to a processing temperature of about 350.degree. C. Since the type of glass in general use in the flat panel display industry has a low CTE (4.6.times.10.sup.-6 /.degree.C.), the size of the glass increases in size only about 0.15% from room temperature to 350.degree. C. Because of this difference, when the glass plate and susceptor are heated to elevated temperatures, there is a significant difference in size between the glass and its susceptor support relative to the room temperature condition and it becomes difficult to center or maintain alignment of the heated glass plate on the susceptor. Again this contributes to non-uniformities in the amount of masking occurring along each edge plates and to unacceptable variations in the location of the deposition zone on the glass plate.
Therefore a means of centering a large glass substrate with respect to its susceptor support and to a shadow frame has been sought.